In a typical turbine, for example, a gas turbine, an annular shroud forms the radially outermost wall surface or flow path surface about the outer tips of rotating blades or buckets in a turbine stage. The annular shroud is typically comprised of a plurality of arcuate segments disposed end-to-end to completely encompass the hot gas flow path. Conventionally, each shroud segment includes forward and rear rails interconnected along radial innermost ends by a flow path section carrying the flow path surface and defining the radial outer limit of the gas flow path. In addition to the flow path section, the forward and rearward rails of each shroud segment have typically been connected to one another by two side walls at the respective opposite circumferential ends of the segment and which essentially extend axially within the turbine shroud. These side walls reinforce the forward and rear rails and, in combination with the rails, define a pocket within the shroud segment which opens radially outwardly.
It will be appreciated that the temperatures in the hot gas flow path of a gas turbine can reach as high as 1600-1700.degree. F. and that the flow path surface of the shroud is exposed to such high hot gas flow path temperatures. However, the forward and rear rails, as well as the side walls, extend radially outwardly of the hot gas flow path and the flow path section of the shroud segment and are therefore subjected to lower temperatures. Consequently, thermal induced stresses within the shroud segments occur as a result of the temperature distribution or gradient about the shroud segment. These induced stresses can cause damage to the shroud segments as well as stress the multiple connections with the turbine shell casing. It will be appreciated that the forward and rear rails of the shroud segments have axially directed flanges or hooks which cooperate with turbine casing hooks to secure the shroud segments to the turbine casing. Thermal stresses on the shroud segments can apply significant forces to the turbine hooks, resulting in high stresses and potential fracture of the turbine casing hooks.
Thermal induced stresses in shrouds have not heretofore been addressed to any large extent. Conventional shroud segments typically have very thick forward and rear rails in comparison with the thickness of the flow path section of the shroud segment. The ratio of the cold mass to the hot mass, i.e., the cold mass of the forward and rear rails and side walls to the hot mass of the flow path section, has been found significant in causing thermal induced stresses having resulting destructive potential.
Furthermore, shroud segments are typically expensive and laborious to manufacture. For example, while continuous turning-type machining of shroud segments is conventional, it is necessary in view of the side walls of the shroud segment to mill the pocket within the segment between the opposite side walls and the forward and rear rails. Necessarily, the milling operations produce thick forward and aft rails which enlarge the cold-to-hot mass ratio. Some shroud segment designs employ a cast-in pocket which, to some extent, reduces the thickness of the forward and rear rails but produces a very expensive design and uses cast material with inferior properties.